Periodic and well-defined features in the nano-scale are essential in several fields. These include photonics, optical applications such as diffraction gratings and antireflection surfaces, nanoelectronics, high-density information storage media, catalysis, bioanalytics, medicine and many others.
The “periodicity” of a pattern formed on a surface can be mathematically rigorously described by performing a circularly averaged Fast Fourier Transform (FFT) of the surface heights, or by calculating the Height-Height Correlation Function (HHCF) of a surface. A completely periodic, deterministic surface will display an approximately delta-function peak in the Fourier transform, and a persistent oscillation in the HHCF. The inverse of the peak position of the FFT is the wavelength or periodicity of the structures.
Self-assembly methods (as defined below) often lead to surfaces exhibiting a peaked FFT, and several oscillations in the HHCF. The narrower the peak in the FFT, and the larger the extent of the oscillations in the HHCF, the more periodic the pattern is. Periodicity is quantified by the full width at half maximum (FWHM) of the FFT. The inverse of this quantity is the system correlation length, which shows the extent of the persistence of order, and should be larger than the wavelength of the periodic structures.
In highly periodic surfaces a sharp narrow peak tends to exist in the FFT and several oscillations in the HHCF. For less periodic surfaces only a broad peak exists in the FFT, and only a few small-amplitude oscillations in the HHCF. In stochastic columnar-like surfaces, no peak is observable in the FFT, nor any oscillations in the HHCF, but one may define an average column diameter and separation distance with a large standard deviation.
Aspect ratio is defined as the height divided by the diameter of a structure. High Aspect Ratio (HAR-HSA) structures are typically defined as those with a value of aspect ratio which is larger than 3 and Low Aspect Ratio (LAR) structures are those which typically have a smaller value of the aspect ratio.
A variety of methods may be employed to attain the desired patterning; the most commonly used being photolithography (“top-down”) techniques. However conventional photolithography schemes, although capable of achieving below 32 nm resolution using 193 nm lithography and resolution enhancement methods, or Extreme Ultra Violet Lithography (EUV) at 13.5 nm, are extremely costly. Other “top down” methods such as electron beam or ion beam lithography are slow step-wise processes.
These drawbacks have motivated the development and implementation of alternative patterning techniques that do not utilise pre-formed/patterned mask or serial writing. Such techniques include: (a) self-assembly approaches to create patterns, or (b) spontaneous formation of patterns in the course of an appropriately tuned process.
Definitions of “self-assembly” vary hugely and generalisation is somewhat difficult1. Self-assembly processes often lead to autonomous organization of components into structurally well-defined patterns, without direct human intervention, either through physical or chemical forces. Also, the self assembly may be assisted by, for example, bio-molecules2, colloidal particles3, Langmuir-Blodgett films4, poly(styrene)-PS nano-spheres5 and block-copolymers6.
The self-assembly processes, however, suffer from a poor degree of control of spatial and size distribution of the resulting nanostructures. Furthermore, the processes are only applicable to limited classes of materials. Additionally, there are often problems with low throughput or low homogeneity in large areas of the pattern. Moreover, by-products are sometimes generated in the wet-chemical preparation, which makes it difficult to keep the surface clean. In addition specific polymer or other materials are often needed to implement self-assembly, such as block-copolymers, or nano-spheres.
Since, among other drawbacks, periodicity cannot be controlled over large areas in the above methods, ad-hoc techniques such as graphoepitaxy (or pattern-directed self-assembly) may be employed. Graphoepitaxy requires the pre-formation of a specific pattern which directs self-assembly for forming highly periodic structures6.
As mentioned above, “spontaneous formation” of patterns may be used as an alternative to self assembly. Spontaneous formation of well organized structures can be directed, for example, by chemical or electrochemical means7 or thermal-mechanical means8. Alternatively, it may be directed by “dry” processes such as laser9 or plasma10 which may lead to patterns that are formed during a particular process, without mask, template, writing procedure or specific pretreatment to induce a pre-pattern.
In all of these processes a significant tuning of parameters is required in order to control the resulting surface morphology. Accordingly, these techniques do not fall into the “self-assembly” category since a particular process has to be implemented, necessitating direct human intervention. The term “process directed self-assembly”, or self-organization may be used.
Of the techniques that induce spontaneous structure formation, “low-pressure plasma processes” provide some unique advantages in that they are dry, low temperature, clean-room compatible, flexible and amenable to mass production/large area/high throughput10. Until now they have been directed towards fabricating periodic, self-organizing ripple-like patterns such as in the case of poly(dimethyl siloxane)-PDMS after oxygen plasma treatment11-13. In this case deformation and periodic waves (ripple-like) occur due to the formation of SiO2 at the interface between the oxygen plasma sheath and polymer surface, under the effect of the induced stress. Because of the specific mechanism involved, this self-organization process takes place only in elastomeric Si-polymers under oxygen-based plasmas. Furthermore, only ripple-like or worm-like structures are produced, as opposed to “bump-” or mound-like morphology. Such a ripple-like morphology may also be induced by pre-stretching, ion-beam crosslinking, and thermally annealing the polymer sample14.
In general, the use of plasmas leads to surface roughening of both organic or inorganic polymers. Even though it is not fully understood how the roughness evolves, oxygen-based plasmas are employed to produce nano-texturing of the surface of many organic polymers. These include polymers such as fluorinated poly(imide)15 and fluorinated poly(ether)16. Applications of oxygen based plasmas include wetting control and superhydrophobic or super-hydrophilic surface fabrication on poly(propylene)-PP17 and on poly(ethylene terepthalate)-PET18 or on Si in the case of photoresist etching19. Oxygen based plasmas may also be employed, for example, for fabrication of superhydrophobic transparent surfaces on poly(methyl methacrylate)-PMMA20, on SU-821 and for fabricating antireflective coatings on PMMA22.
In WO0402480523 there is described a method for reducing boundary surface reflection of plastic substrates. Oxygen based plasmas may also be employed to enhance the protein and cell adsorption of PMMA24 and for electro-osmotic flow tuning in PMMA microfluidics25.
In WO/2007/03179926 there is described a plasma based method for the fabrication of High-Aspect Ratio and High Surface Area (HAR-HSA) columnar-like structures on any organic and inorganic polymer using appropriate plasma chemistry. In particular a method is described for the control of the wetting properties of surfaces, the flow in microchannels and the separation in microchannels.
However in the aforementioned cases plasma-induced nano-texturing delivers random columnar-like morphology and not well-defined, periodical, self-organized structures with controlled geometrical characteristics. Thus the resulting structures do not ideally serve as alternatives to structures derived from self-assembly techniques. In addition, although there are methods22, 23 describing how to utilise ions resulting from plasmas in reducing reflectivity, there is often a lack of control of the geometrical characteristics and thus the precise broadband reflectance spectrum obtained. In addition, in methods described to date the plasma energies employed may lead to sample heating effects.
Another method to produce self-organised nanopatterns is ion-beam sputtering which may lead to nano-mound or nano-ripple formation on many substrates such as Si27, GaSb28, and InP, InAs29, 30. This method has been recently reviewed by Garcia31. This method suffers from the drawback of need of very high energy ions 300-2000 eV which may cause severe damage to the material, may heat it up, and is thus inappropriate for polymers. In addition it is an extremely slow process requiring hours of exposure to ion bombardment.
Finally there is recently a need to create periodic nanoholes or nanodots or nanopillars of metals to enhance surface Plasmon resonance effects, and thus affect the optical behaviour of thin metal films32, 33, a field with vast applications from solar photovoltaic electrodes to nanoatnennas.
Embodiments of the techniques we describe provide a step towards solutions of these problems.